There has been proposed a light-emitting diode (LED) projector that uses a LED as a light-emitting element included in the light source device. The LED projector of this type includes a light source device having a LED, an illumination optical system into which light from the light source device enters, a light bulb having a liquid crystal display plate into which light from the illumination optical system enters, and a projection optical system for projecting light from the light bulb to a projection surface.
In the LED projector, to increase the luminance of a projected image, light loss must be prevented as much as possible on an optical path from the light source device to the light bulb.
As described in Nonpatent Literature 1, there are restrictions based on Etendue that are determined by the product of the area of the light source device and the radiation angle. In other words, the light from the light source device is not used as projection light unless the value of the product of the light-emitting area of the light source device and the radiation angle is set equal to or less than the value of the product of an incident surface area of the light bulb and a capture angle (solid angle) determined by the F number of a projection lens.
Thus, in the light source device that includes the LED and an optical element into which light from the LED enters, reducing the light loss by lowering the etendue of light output from the optical element is an issue that requires attention.
In the light source device included in the LED projector, it is essential to achieve a projected luminous flux of about several thousand lumina by using a plurality of LEDs to compensate for a shortage of the amount of light emitted by a single LED.
As an example of such a light source using a plurality of LEDs, as shown in FIG. 1, Patent Literature 1 discloses a light source unit that includes a plurality of monochromatic light source devices 83a to 83f having LEDs 84a to 84f, optical axis matching members 82a to 82d for matching optical axes of lights output from monochromatic light sources 83a to 83f, light source sets 81a and 81b into which lights from optical axis matching members 82a to 82d enter, and light guiding device 80 into which the lights from light source sets 81a and 81b enter. In this light source unit, lights from the plurality of monochromatic light source devices 83a to 83f are synthesized, and lights that have radiation angles narrowed by light source sets 81a and 81b enter light guiding device 80. In this configuration, light loss is reduced by narrowing the radiation angles of the light incident on light guiding device 80.
As another example of the light source device using the plurality of LEDs, as shown in FIG. 2, Patent Literature 2 discloses a light source device that includes light source substrate 86 having a plurality of LEDs 85 arranged on a plane. This light source device includes an optical element having two prism sheets 88 and 89 each having prism columns formed on one surface and arranged by setting the prism columns to intersect reach other, and frame 87 for supporting prism sheets 88 and 89. In the light source device, lights from the plurality of LEDs 85 are synthesized by two prism sheets 88 and 89.